In time-of-flight mass spectrometers (TOFMS), a mass sample to be analyzed is ionized, the resulting ions are accelerated in a vacuum by an electrical pulse having a known potential, and the flight times of the ions of different masses at an ion detector are measured. The more massive the ion, the longer is the flight time. The relationship between the flight time and the mass, m, of ions of a given mass can be written in the form:time=k√{square root over (m)}+c where k is a constant related to flight path and ion energy, and c is a small delay time that may be introduced by the signal cable and/or detection electronics. When the term mass is used in this disclosure in the context of mass spectrometry, it is to be understood to mean mass-to-charge ratio. The process of accelerating the ions of the mass sample and detecting the arrival times of the ions of different masses at the ion detector will be referred to herein as a mass scan operation.
The ion detector generates electrons in response to ions incident thereon. The electrons constitute an electrical signal whose amplitude is proportional to the number of electrons. There is only a statistical correlation between the number of electrons generated in response to a single ion incident on the ion detector. In addition, more than one ion at a time may be incident on the ion detector due to ion abundance.
In the mass spectrometer, an ion accelerator generates a short pulse of ions by applying an electrical pulse having a known voltage to ions received from the ion source. Immediately after leaving the ion accelerator, the ions are bunched together but, within the ion pulse, ions of different masses travel at different speeds. The flight time required for the ions of a given mass to reach the ion detector depends on the speed of the ions, which in turn, depends on the mass of the ions. Consequently, as the ion pulse approaches the ion detector, the ion pulse is separated in space and in time into discrete packets, each packet containing ions of a single mass. The packets reach the ion detector at different arrival times that depend on the mass of the ions therein.
The mass spectrometer generates what will be referred to a mass scan signal in response to a single pulse of ions accelerated by a single electrical pulse. The mass scan signal is a digital signal that represents the output of the ion detector as a function of time. The time represents the time-of-flight of the ions from the ion accelerator to the ion detector. The number of electrons generated by the ion detector in a given time interval constitutes an analog ion detection signal that is converted to the mass scan signal by an analog-to-digital converter (A/D converter). The mass scan signal represents the output of the ion detector as a function of the flight time taken by the ions to reach the ion detector. The mass scan signal is a temporal sequence of digital samples output by the A/D converter after the ions have been accelerated. The conversion time of the A/D converter effectively divides the time axis into discrete bins and the A/D converter outputs a single digital sample for each bin on the time axis.
Because the relationship between the amplitude of the ion detection signal output by the ion detector and the number of ions incident on the ion detector is a statistical one, a single mass scan signal will not accurately represent the mass spectrum of the sample. In addition, the ion detection process is subject to noise from a number of different noise sources. Such noise causes the ion detector to generate an output signal even in the absence of ions incident on the ion detector. To overcome these problems, the mass spectrometer generates multiple mass scan signals and sums the most-recently generated mass scan signal with an accumulation of all previously-generated mass scan signals to generate a mass spectrum having a defined statistical accuracy and signal-to-noise ratio.
The resulting mass spectrum is subject to mass resolution limitations originating from the ion accelerator and the ion detector and its associated circuitry. The mass spectrometer and mass spectrometry method disclosed in the parent application decreased the mass resolution limitations originating from the ion detector and its associated circuitry leaving the ion accelerator as the primary limiter of mass resolution. This has prompted improvements in the precision of the mass accelerator so that, once more, the ion detector and its associated circuitry have become contributors to mass resolution limitations.
Accordingly, what is needed is to reduce the mass resolution limitations imposed by the ion detector and its associated circuitry.